For several months after the US entry into World War II, Americans feared air raids or even invasions of the continental United States by Germany and Japan. An article in the March 2, 1942 issue of Life magazine suggested several possible invasion routes that Axis forces might take. These include a Germany landing at Norfolk, Virginia supported by fifth columnists hidden in the US. At the link, you can view maps of a few other fanciful scenarios.

I think these ideas weren't so far fetched in view of that time. Some years earlier the strategists at the war department had research done and plans laid out for war scenario's with Canada in case the UK would lose a war in Europe. By 1942, talk in Nazi Germany was all about Things to Come in the 'You wait and see'-speeches of the Führer and his propaganda-machine. In 1942, the German U-boats were patrolling and sinking ships in sight of tha mainland of the US. So the threat of Germans at the footstep of the US was felt very real. And so while in the early 1940's one part of the strategists in the US was correctly calculating that the war would last only a few years more, others just had to count with the possibility that the Axis Forces had some hidden trumpcards up their sleeves to invade or at least attack the mainland of the US. And they all realised that those fears could be used to rally the public.

I think you are over estimating the logical processing of the general public. The military obviously had little doubt that a direct attack on the US by Germany was extremly unlikely given that they build beach watch towers only around major port paths and having German POWs located in east coast towns.

I am just saying that the general public does not always take the logical path when fear is involved. I bet it would be very easy to find people that believe Russia could invade the US within a few months. Even easier if some major website posted "plans" of such a things even if it was a joke.

Cluck, after the battle of Britain Hitler called off his plans to invade Britain. Even so without that particular item of knowledge it's still pretty clear that an invasion of the US would be almost impossible.

The commitment of equipment and manpower to achieve such a thing would have been enormous and given that the Germans were already heavilly comitted on multiple other fronts it would have been clear to anybody in military intelligence that such an invasion was not going to happen.